netdev - Re: [PATCH rfc bpf-next 7/8] bpf, x86: emit patchable direct jump as tail call

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Date:   Fri, 15 Nov 2019 08:27:38 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     ast@...nel.org, john.fastabend@...il.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH rfc bpf-next 7/8] bpf, x86: emit patchable direct jump as
 tail call

On Thu, Nov 14, 2019 at 07:23:46PM -0800, Alexei Starovoitov wrote:
> On Fri, Nov 15, 2019 at 02:04:01AM +0100, Daniel Borkmann wrote:
> > for later modifications. In ii) fixup_bpf_tail_call_direct() walks
> > over the progs poke_tab, locks the tail call maps poke_mutex to
> > prevent from parallel updates and patches in the right locations via
> ...
> > @@ -1610,6 +1671,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> >  		prog->bpf_func = (void *)image;
> >  		prog->jited = 1;
> >  		prog->jited_len = proglen;
> > +		fixup_bpf_tail_call_direct(prog);
> 
> Why not to move fixup_bpf_tail_call_direct() just before
> bpf_jit_binary_lock_ro() and use simple memcpy instead of text_poke ?

Thinking about it, I'll move it right into the branch before we lock ...

  if (!prog->is_func || extra_pass) {
    bpf_tail_call_fixup_direct(prog);
    bpf_jit_binary_lock_ro(header);
  } else { [...]

... and I'll add a __bpf_arch_text_poke() handler which passes in the
a plain memcpy() callback instead of text_poke_bp(), so it keeps reusing
most of the logic/checks from __bpf_arch_text_poke() which we also have
at a later point once the program is live.

> imo this logic in patch 7:
> case BPF_JMP | BPF_TAIL_CALL:
> +   if (imm32)
> +            emit_bpf_tail_call_direct(&bpf_prog->aux->poke_tab[imm32 - 1],
> would have been easier to understand if patch 7 and 8 were swapped.

Makes sense, it's totally fine to swap them, so I'll go do that. Thanks
for the feedback!

Cheers,
Daniel